1. Field of the Invention
The present invention relates to a sliding bearing interposed between, for example, a steering column tube and a steering column shaft to rotatably support the steering column shaft of an automobile, a sliding bearing interposed between a rack shaft and a tubular member so as to support the rack shaft linearly movably, and a bearing mechanism having such a bearing.
2. Description of the Related Art
As a bearing for a steering column for rotatably supporting a steering column shaft (hereafter referred to as the column shaft) of an automobile, a rolling bearing constituted by a ball bearing or a sliding bearing formed of a synthetic resin is used.
In general, conditions for rotation such as a load and a velocity are not so stringent for the bearing for rotatably supporting the column shaft. However, the vibration absorbing characteristic for absorbing vibrations applied to the column shaft and stability in frictional torque are required at the time of idling or the like.
The rolling bearing is stable in the frictional torque, but is inferior in the vibration absorbing characteristic. In addition, the dimensional accuracy of a steering column tube (hereafter referred to as the column tube) for fixing the rolling bearing and the column shaft supported by the rolling bearing need to be finished to high accuracy. Therefore, on top of the fact that the rolling bearing itself is expensive, there is a problem in that the fabrication cost becomes high.
The sliding bearing has advantages in that, as compared with the rolling bearing, the sliding bearing is low in cost and excels in the vibration absorbing characteristic. Nevertheless, there is a problem in that since an appropriate clearance (axial gap) is required between the sliding bearing and the column shaft, collision noise occurs between the column shaft and the bearing, and is transmitted to the driver of the automobile as unpleasant sound. If the clearance between the sliding bearing and the column shaft is made small to suppress the occurrence of this collision noise, the frictional torque increases, and the difference in frictional torque between at the time of rotation start and during rotation becomes large. At the same time, the small clearance constitutes a factor hampering the stability in frictional torque, such as the occurrence of variations in frictional torque during rotation due to a stick-slip phenomenon or the like caused by dimensional errors in the outside diameter of the column shaft.
In addition, although the column shaft is rotatably supported by the column tube by means of the bearing, the roundness of the inside diameter of the column tube is normally not very high. If the sliding bearing made of a synthetic resin is press fitted and fixed into such a column tube, the sliding bearing is strained by being affected by the roundness of the inside diameter of the column tube, producing a difference in the clearance with the column shaft. This also hampers the stability of the frictional torque. (Refer to JP-A-11-201154 and JP-UM-B-56-39747 as prior arts)
The above-described problems occur not only in the sliding bearing interposed between the column shaft and the column tube which rotatably supports this column shaft, but can similarly occur, for example, in a bearing interposed between a rack shaft and a tubular member (tube) which supports the rack shaft linearly movably.